1. Field of the Invention:
This invention relates to a projection type cathode-ray tube having an optical multilayered interference film, and more particularly to a projection cathode-ray tube which prevents a discoloring phenomenon (hereinafter called as "browning") of the inner surface of a face panel.
2. Description of the Related Art:
A first related art is exemplified by U.S. Pat. No. 4,642,695 which is owned by the inventor of this invention. This U.S. Pat. No. 4,642,695 discloses a method for improving the low efficiency of gathering luminous flux into a projection lens unit from respective monochromatic projection cathode-ray tubes in a projection type television set.
In practice, in an ordinary cathode-ray tube, although the luminous flux emitted from a phosphor screen is nearly a so-called perfectly diffused light, among the luminous flux emitted from the phosphor screen only the luminous flux in the region having a divergent angle of +/-30 degrees is converged into the projection lens unit and is utilized effectively, while the remaining luminous flux becomes disregarded.
This disregarded luminous flux is reflected by a tube mirror and turned to be a stray light, impairing the contrast of the projected image. This first related art being set forth above aimed to overcome the above-mentioned drawbacks, whereby it became possible to enhance the brightness of an image on a screen of the projection type television set by converging the luminous flux in the excess 30% of total luminous flux emitted from an emission point on the phosphor screen into a cone having the divergent angle of +/-30 degrees.
To achieve the aim of the above-described first related art, another related art is exemplified by Japanese Patent Publication Laid-open No. 60-257043 also filed by the same inventor.
This second related art discloses a projection cathode-ray tube having a plurality of optical multilayered interference films composed of a plurality of alternately superimposed layers of a high-refractive-index film and a low-refractive-index film, and proposes the use of the optical multilayered interference film composed of six high-refractive-index layers consisting of tantalum oxide (Ta.sub.2 O.sub.5) and the low-refractive-index layers consisting of silicon oxide (SiO.sub.2). According to this second related art, it is possible to realize an angular distribution of brightness in luminous flux of the phosphor screen, and consequently a high quality projection cathode-ray tube can be obtained.
However, the following two drawbacks have been found in conjunction with the above-stated related art.
Specifically, regarding the second related art, in spite of the foregoing advantages, one drawback has been that the output of luminous flux emitted from the projection cathode-ray tube having the multilayered interference film is very much decreased as operating time elapses as compared with the deterioration that occurred in the projection cathode-ray tube without the optical interference film.
A rate of deterioration in output of the luminous flux emitted from the cathode-ray tube will now be explained.
FIG. 2 of the accompanying drawings illustrates a variation of the output of the luminous flux with the elapse of operating time when a projection cathode-ray tube for a green luminous flux is continuously operated at a high voltage (acceleration voltage) of 32 kV and a current density of 6 .mu.A/cm.sup.2 on the phosphor screen. Here, assume that in either case an outer surface of the face panel of the projection cathode-ray tube is cooled by a coolant.
In FIG. 2, a curved line III is a line representative of deterioration in light output of the projection cathode-ray tube without the optical multilayered interference film and shows that the output of the luminous flux is decreased to 74% of the initial output with the elapse of 7,000 hours of operating time.
As major factors of this deterioration phenomenon, there are enumerated a degradation in luminous efficiency of phosphors and a discoloring phenomenon known as browning of the inner surface of the face panel.
As of yet, each of these factors is considered to contribute to this deterioration at a ratio of fifty-fifty. Column A of table 1, as will be described later, shows a rate of deterioration in light output due to the degradation in phosphors and a rate of deterioration in light output due to the browning discoloration of the inner surface of the face panel, respectively. In this table, the initial value is defined as 100%, and each value is represented by a ratio of a light output value to the initial light output defined as 100%.
As is evident from the result shown in the table, it is considered that the degradation in luminous efficiency of the phosphors is caused by the gradual destruction of the luminance mechanism of the phosphors due to the energy of the electron bombardment and due to heat and X-rays caused when the electrons collide.
The browning discoloration is substantially classified into two types, that is, an electron browning and an X-ray browning.
The former browning occurs by alkali metal ions such as sodium (Na) and potassium (K), which constitute the face panel, which are reduced and metalized by the energy caused when the electrons which traveled through the gap in the phosphor layer directly collide with the inner surface of the face panel.
The latter browning is a kind of solarization, and is caused by the occurrence of a discoloring center at a lattice defect in the surface glass of the face panel due to the X-ray energy generated when the electrons make a collision with the phosphor screen and the glass surface at high velocity.
Both the electron browning and the X-ray browning cause the glass of the face panel to be discolored. As is apparent from FIG. 3, a spectral transmissivity distribution (b), after discoloration, shows a steeper slope of the transmissivity curve in the shorter wavelength region of visible light as compared with a spectral transmissivity distribution (a) before discoloration.
A curved line II in FIG. 2 represents a slope of degradation in light output of the projection cathode-ray tube (conventional type 2) having the optical multilayered interference film.
In the structure of the conventional cathode-ray tube (2) as shown in FIG. 4, the face panel 1 has on its inner surface the optical multilayered interference film 2 made up of five thin alternately superimposed layers of a high-refractive-index film of titanium dioxide (TiO.sub.2) and a low-refractive-index film of silicon dioxide (SiO.sub.2), and the phosphor layer 3 and the metal back layer 4 are disposed over the multilayered interference film.
As described above, in accordance with the conventional projection cathode-ray tube 2, as can be seen from the curved line (II) of FIG. 2, the light output dropped to 63% of the initial light output value with the elapse of 7,000 hours of operating time, and the curve of degradation in light output is far steeper than the slope of the curved line (III) of the foregoing conventional projection cathode-ray tube 1. A factorial experiment of this result is illustrated in column B of the table 1.
Naturally, since the presence of the optical multilayered interference film has no correlation with the degradation of the phosphors, the light output of the projection cathode-ray tube in accordance with the present invention has the same value as that of the conventional projection cathode-ray tube 1 without the optical multilayered interference film.
Further, the optical multilayered interference film itself is subjected to the browning, and consequently the light output of the cathode-ray tube is dropped by 5%. Here, attention should be given to the fact that the decrease in light output is due to the browning on the glass surface.
Namely, in the case of the conventional projection cathode-ray tube 1 without the optical multilayered interference film, the drop rate of the light output from the cathode-ray tube due to the browning on the glass surface of the face panel is 14%, whereas that of the conventional cathode-ray tube 2 having the optical multilayered interference film is 23%.
Thus, the light output is much deteriorated by the cathode-ray tube having the multilayered interference film as compared with the deterioration by the cathode-ray tube without the multilayered interference film.
Originally, the optical multilayered interference film coats the glass surface and serves to weaken the energy of the electrons which collide with the glass surface. Accordingly, the browning discoloration of both the electron browning and the X-ray browning is subsequently expected to be diminished.
However, as seen from the result in the table 1, in the case of the conventional cathode-ray tube 2 having the optical multilayered interference film, the browning on the glass surface of the face panel is conversely increased.
In the study of causes of the increase of browning in the conventional projection cathode-ray tube 2 having the optical multilayered interference film, it is found that browning of the glass surface of the face panel is increased by a mechanism, as will be described later.
In short, in the case of the conventional cathode-ray tube 2, as shown in FIG. 4, the optical thin film layer of high-refractive-index of titanium dioxide (TiO.sub.2) is deposited on the glass surface of the face panel 1 as a first optical layer.
Since the optical multilayered interference film 2 set forth has five layers and has a thickness of 0.5 to 0.7 micrometer, the electrons travelling through the gap of the phosphor screen 3 penetrate through the optical multilayered interference film 2 and can reach the region of the glass surface of the face panel 1.
During this time, the optical thin film layer of titanium dioxide (TiO.sub.2), formed over the glass surface of the face panel 1, is subjected to the electron bombardment, and consequently titanium dioxide (TiO.sub.2) is reduced to titanium monoxide (TiO) by the removal of an oxygen (O) therefrom. The titanium monoxide (TiO) is highly unstable and acquires oxygen (O) from the glass surface of the face panel 1 so as to be a stable titanium dioxide (TiO.sub.2).
Since sodium oxide (Na.sub.2 O) and potassium oxide (K.sub.2 O) are present in the form of ions, sodium ions and potassium ions are turned into a sodium metal and a potassium metal by a reducing reaction when oxygen (O) is removed. With this result, the browning discoloration is considered to be accelerated. Particularly, when as in many cases, the first layer of the high refractive index film is made from metal oxides.
Through a research of various metal oxides practicable in view of their optical property, it was found in more or less all metal oxides studied that a browning discoloration occurs to some extent.